As used herein, the term "prop rotor" refers to a system that rotates and provides lifting and control forces. The term "rotor blade" refers to the aerodynamic, structural, and lifting portion of a rotor system that directly connects to a central hub or drive. Rotor blades include an elongated airfoil-shaped wing, a blade shaft, and in some embodiments, a blade spindle. Rotating blades provide lift and propulsive force for helicopters and tilt rotor aircraft during all or some portion of their flight regime. The number of blocks directly affects the vibratory frequency of the rotor system and, together with the rotational speed, also significantly influences the acoustical signature.
The operational forces experienced by a prop rotor include aerodynamic, inertial, and centrifugal forces. These forces produce complex and highly stressed loadings on the rotor blades as well as the central hub to which the blades are attached. To control the craft, mechanisms are commonly provided to rotate the blades about a blade longitudinal axis during aerodynamic loading in order to control the pitch of the blades. To accept system structural stresses through the entire spectrum of control inputs, it is important that all system components be designed to the most optimum dynamic properties, including blade natural frequencies and in-plane (lead-lag) and out-of-plane (flapping) deflections and frequencies.
Known articulated rotor hub designs incorporate various mechanical flapping hinges and lead-lag hinges in order to reduce structural stresses while allowing for aerodynamic control of the rotor systems. Although lead-lag and flapping hinges help reduce stresses, they also increase the complexity of the overall structural design and, in the case of lag hinges, can contribute to degradation of the aerodynamic stability at high speeds. In response to these problems, designers have created rotor system designs that incorporate the use of flexible beams or other flexible structures directly into the design of the hub, in place of the mechanically complex lead-lag or flapping hinges, and have also created rigid rotor systems, having no flexible hinges.
Two such flexible beam designs are described in U.S. Pat. Nos. 5,096,380 and 5,372,479 both issued to Byrnes. These designs both eliminate the need for lead-lag hinges and flapping hinges by connecting a rotor blade to the hub using a flexible composite beam. The flexible composite beam directly connects the rotor blade to the rotor assembly while allowing both inboard flapping flexure and outboard lead-lag and torsion flexure. The thickness and dynamic characteristics of the flex beam are designed to allow a suitable degree of lead-lag and flapping flexure. Although the use of a single flexure as set forth in the Byrnes patents results in decreased complexity, it can also result in a potential decrease in rotor and blade stability at high speeds.
Thus, a need exists for a rotor system design that reduces complexity while maintaining high speed rotor stability. The ideal design would further be of minimum weight. The present invention is directed towards addressing this need.